Pitch signal calculator for aircraft

ABSTRACT

An instrument for calculating the pitch angle of an aircraft from two pitch signals, one having long term and the other short term pitch accuracy. The long term pitch signal is derived from a longitudinal accelerometer. A measured pitch signal having short term accuracy is provided by a vertical gyroscope. An airspeed acceleration signal is provided by a vertical gyroscope. An airspeed acceleration signal is subtracted from the accelerometer output to eliminate the acceleration component of the signal. The calculated and measured pitch signals are combined or blended in selected inverse proportions.

United States Patent [191 Astengo [451 July 10, 1973 [54] PITCH SIGNALCALCULATOR FOR Primary Examiner-Donald O. Woodiel AIRCRAFTAtt0rney-Hofgren, Wegner, Allen, Stellman & Mc- [75] lnventor: Ralph A.Astengo, Bellevue, Wash. Cord [73] Assignee: Sundstrand Data Control,Inc., [57] ABSTRACT Redmond? wash An instrument for calculating thepitch angle of an air- 2] ed July 1970 craft from two pitch signals, onehaving long term and 21 A L N 56,61 the other short term pitch accuracy.The long term 1 DP 0 3 pitch signal is derived from a longitudinalaccelerometer. A measured pitch signal having short term accura- [52]US. Cl. ..73/178 R cy is provided by a vertical gyroscope. An airspeedac- [5l] Int. Cl. ..G0lc' 23/00 l ratio signal is provided by a verticalgyroscope. Fleld of Search 178 173 173 l An airspeed acceleration signalis subtracted from the accelerometer output to eliminate theacceleration 56 R t component of the signal. 1 e erences Cited Thecalculated and measured pitch signals are com- UNITED STATES PATENTSbined or blended in selected inverse proportions. 3,371,528 3/1968Sullivan ..73/l78 T 6 Claims, 5 Drawing Figures /2 h /& LONGITUDINAL LLow PASS I S'GNAL ACCELEROMETER I FILTER I i 'i E L l l L I 20/ I I /422 I I I VERTICAL g I HIGH PASS GYRO i FILTER I 24 2 AIR SPEED v I BANDPASS SENSOR FILTER PAIENIEDJUHOW 3.144309 /2 I I" l "/5 LONGITUDINAL I.I LOW PASS l ACCELEROMETER I FILTER I I B' EN DER L I I [-0 I 20/ I I II /4 l 22 25 l K I I I I 6 e VERTICAL g I HIGH PASS i I 99 GYRO I FILTERI l T I I I 699""; I Y I ,6 I I 5 I I 26 I aa L I I I u fi AIR SPEED v IBAND PASS I 34 SENSOR I FILTER I I l f J A /'V a 6 0m) 69 mm) I T l I o.L A w I fl-w. w) K 3 Uw) 1 aecuw) I m. av (I00) I l o i w T m Y 69 A KI INVENTOR 5 9 25? I I -I $278,390 a I I f av M I I foz q 0 T ATTORNEYS1 PITCH SIGNAL CALCULATOR FOR AIRCRAFT BACKGROUND OF THE INVENTION Thisinvention relates to an instrument for computing or calculating thepitch angle of an aircraft.

Vertical gyroscopes have commonly been used to derive the pitch angle ofan aircraft for use in various instrumentafion. However, the output of agyroscope, when uncorrected, has not always proven adequate,particularly when long term pitch information is needed. Ideally, agyroscope remains fixed in inertial space, but in practical usage it issubject to numerous types of errors. Initially, there is a likelihoodthat the gyroscope will be mounted with some misalignment from thedesired position. In addition, constructional deficiencies and designlimitations cause undesired precession of the gyroscope with resultingdrift errors, both fixed and random. For example, constraint errorsoccur due to torques induced by the elasticity of power leads and thereaction of gyroscope pick-offs or signal transducers. If the gyroscoperotor is not precisely balanced about its precession axis, a massunbalance error occurs under the influence of gravity or otheracceleration. Another source of gyroscope drift error, termedanisoelasticity, is caused by unequal deformations of the gyroscopesupport structure when load is applied from different angles. Variousgyroscope errors, such as those listed above, cause degradation of theaircraft systems performance. Although some of these errors,particularly those of a fixed nature, can be partially eliminated bycalibration and the application of biases during operation of thegyroscope, the procedures are generally time-consuming and costly, andoften require use of expensive calibration calibration apparatus, suchas a computer.

SUMMARY OF THE INVENTION A principal feature of the present invention isthat an instrument is provided for computing the pitch angle from asource of measured aircraft pitch angle, while eliminating errorsassociated with the pitch source. More particularly, the instrumentincludes a source of acceleration along the longitudinal axis of theaircraft, a source of measured aircraft pitch angle, and a source ofaircraft air speed. A complementary filter is connected to the sourcesand summing means is connected to the filter from which a computed pitchsignal is established.

Another feature of the invention is that the complementary filterincludes a low pass filter connected with the source of aircraftacceleration, with the air speed sources and substitute therefor-a highpass filter connected with the measured pitch source and a band passfilter connected with the air speed source.

A further feature of the invention is that the calculated and measuredpitch signals are blended together in inverse proportions as may bedesirable, depending on the application or end use of the pitch signal.

Further features will become more fully apparent in the followingdescription of the embodiment of this invention and from the appendedclaims.

DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a functional block diagram of the instrument of the presentinvention;

FIG. 2 is a curve showing the relationship of a change in calculated tomeasured pitch angle as a function of frequency;

FIG. 3 is a curve showing the relationship of a change in calculatedpitch angle to a change in longitudinal acceleration as a function offrequency;

FIG. 4 is a curve showing the relationship of a change in calculatedpitch angle to a change in air speed as a function of frequency; and

FIG. 5 is a curve showing the relationship of blended to measured pitchangle as a function of frequency for difierent gain constants used inthe signal blender.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring no to FIG. 1, anaircraft instrument, generally designated 10, is shown which calculatesthe pitch angle of an aircraft. A longitudinal accelerometer 12 providesa signal representing acceleration, A alongthe axis of the aircraft, avertical gyroscope 14 provides a signal representing measured aircraftpitch angle, 0,,, and an air speed sensor 16 provides a signalrepresenting aircraft air speed, V.

The acceleration, pitch and air speed sources are connected to acomplementary filter, generally designated 18. In the filter 18, a lowpass filter 20 is connected with the source of longitudinal accelerationA a high pass filter 22 with the source of measured pitch angle 0,, anda band pass filter 24 with the source of air speed V. The outputs offilters 20, 22 and 24 are combined at summing point 26 and a signal isestablished representing a computed pitch angle, 0

An accelerometer utilizing a moving mass has an axis along whichacceleration is sensed. If the accelerometer is accelerated in adirection oblique to its sensing axis, only the component of theacceleration along the sensing axis is indicated; if the accelerometeris moved in a direction perpendicular to its sensing axis, noacceleration will be indicated. Thus, the acceleration sensed by anaccelerometer is a function of the angle between its sensing axis andits direction of acceleration. In the present instrument, thelongitudinal accelerometer in an aircraft has an output which is afunction of both the forward acceleration or change in aircraft forwardspeed and the aircraft pitch angle, through which the acceleration ofgravity acts.

The deficiencies of the measured pitch signal from the verticalgyroscope, particularly the static offset and long termdrift, have beendescribed above. On the other hand, the long term pitch component of theaccelerometer signal is, if the accelerometer is properly installed andadjusted with respect to the airframe, free of these problems. The pitchsignal from the vertical gyroscope, however, includes an' accurate highfrequency pitch signal. The pitch calculating instrument of thisapplication makes use of these signal characteristics to calculate anaccurate pitch signal, free of the long term inaccuracies of thevertical gyroscope measured pitch signal.

The output signal from the longitudinal accelerometer is made up of twocomponents, one representing forward acceleration and the otherrepresenting pitch angle. In order to utilize the pitch angle component,it is necessary to separate it from the forward acceleration signal. Theoutput of air speed sensor 16 is a signal, V, representing the aircraftvelocity. The sensor may,

for example, be a pitot system which determines velocity by comparingdynamic air pressure with static air pressure. The air speed signal iscoupled to band pass filter section 24, the output of which representsthe rate of change of air speed, a signal which is equivalent to theforward acceleration signal from longitudinal accelerometer 12. The airspeed acceleration signal from the air speed sensor is subtracted fromthe output of low pass filter 20 at summing junction 26 and theresulting signal is a function essentially only of the long term pitchattitude of the aircraft.

The output of vertical gyroscope 14 is connected through high passfilter 22 to eliminate the static and long term portions of the measuredpitch signal. The filter output signal is a function essentially only ofthe short term pitch attitude of the aircraft. The short term pitchinformation, which is missing from the output of low pass filter 20, isconnected with summing junction 26. The output from the summing junctionis a calculated pitch signal, 0,; having long term, low frequencyinformation form the longitudinal accelerometer 12 and short term orhigh frequency information from the output of vertical gyroscope 14.

In a representative system, the three filter sections 20, 22, and 24 ofcomplementary filter 18 might have the transfer function set forth inthe following table:

I Low pass fi ter 2 r s 21's High pass f1 ter 22 Band pass filter 24 mwhere K 19 kts/sec/g The output of longitudinal accelerometer 12 may berepresented as The output of low pass filter 20 is The output ofvertical gyroscope 14 is a measured pitch signal. The output of highpass filter 22 is (-r s 21's) (rs 1 The air speed output V of sensor 16is coupled to band pass filter 24 which has an output of the followingform 57.3 ss7.3 1

K Ts+1 K (Hg The calculated pitch signal 6 from summing junction 26 maybe expressed as follows Band pass filter 24 derives from the air speedsignal V a signal representing air speed changes which effectivelycancels the forward acceleration component of the output of longitudinalaccelerometer 12.

FIGS. 2, 3, and 4 illustrate graphically some of the characteristics vofthe calculated pitch signal with respect to the system inputs, as afunction of frequency. In FIG. 2 the ratio of measured pitch tocalculated pitch is plotted. At zero frequency, i.e., steady state orstatic pitch measurement, the measured pitch signal 6,, from thevertical gyroscope does not appear in the calculated pitch signal. Thecontribution of the vertical gyroscope reaches a maximum at a frequencyof l/r and then decreases to the frequency 2/1'. At higher frequenciesthe contribution of the vertical gyroscope signal is constant and, aswill appear, provides essentially the entire calculated pitch signal.

In FIG. 3, the differential relationship of calculated pitch to theoutput of the longitudinal accelerometer is plotted. The relationship isconstant from zero frequency to l/r and then decreases rapidly to zero.The calculated pitch signal does not include any high frequencycomponents of the longitudinal accelerometer output.

In FIG. 4 the differential relationship of the calculated pitch signalto air speed is shown. At zero frequency air speed is not a factor. Itreaches a maximum at a frequency U1 and again decreases to zero,complementing the forward acceleration component of the output of thelongitudinal accelerometer.

The calculated pitch signal 0 is free of steady state and long termerrors in the vertical gyroscope. Under most flight conditions 6,;provides a very accurate pitch signal. However, in a condition of windshear, where the wind velocity varies with altitude, a plane which isclimbing or descending will experience an error in the calculated pitchsignal because the shear causes an inertial acceleration or decelerationwithout a change in air speed.

The instrument illustrated in FIG. 1 includes a signal blender 30 whichpermits the selection of either the measured pitch signal 6,, from thevertical gyroscope, the calculated pitch signal 6 or a combination ofthe two signals. The signal blender comprises two variable gainamplifiers 32 and 34 connected with the outputs of summing junction 26and vertical gyroscope 14, respectively. The outputs of the twoamplifiers are combined at summing junction 36 providing a blended pitchsignal, 0 Amplifier 32 has a gain factor of K while that of amplifier 34has a gain factor of lK. The K gain factor for the two signals may bepreselected by the manipulation of control 38.

The operation of the blender is illustrated graphically in FIG. 5 wherethe ratio of the blended pitch output signal 0 to measured pitch 0,, isplotted as a function of frequency for various values of K. When Kequals zero, 6,, 0,, and the ratio is I. When K l, the ratio is the sameas that of FIG. 2. For intermediate values of K, the ratio assumes anintermediate value.

Iclaim:

l. A pitch angle generator for an aircraft, c0mpris mg:

a longitudinal accelerometer having an output which is a function of theforward acceleration and the pitch attitude of the aircraft;

a vertical gyroscope having an output which represents the measuredpitch of the aircraft, subject to static and long time drift errors;

first means for deriving from the output of the longitudinalaccelerometer a signal which is a function essentially only of theforward acceleration and the long term pitch attitude of the aircraft;second means for deriving from the output of the vertical gyroscope asignal which is a function essentially only of short term pitch attitudeof the aircraft;

third means for developing a signal representing forward acceleration ofthe aircraft; and

combining means for subtracting the forward acceleration signal from thesignal derived by the first means and adding said long term and shortterm pitch attitude signals to generate a calculated pitch signal.

2. The pitch angle generator of claim 1 in which the means fordeveloping a signal representing forward ac celeration of the aircraftincludes means for measuring the air speed of the aircraft and means forestablishing a signal representing the rate of change of the air speed.

3. The pitch angle generator of claim 1 in which the two signal derivingmeans form a complementary filter having a low pass filter sectionconnected with the output of said longitudinal accelerometer and a highpass filter section connected with the output of said verticalgyroscope.

4. The pitch angle generator of claim 3 in which the means fordeveloping a signal representing a forward acceleration of the aircraftincludes means for measuring the air speed of the aircraft and means,including a band pass filter section, for establishing a signalrepresenting the rate of change of air speed, which is combined with theoutput of said low pass filter section.

5. The pitch angle generator of claim 1 including means for summing asignal representing the measured pitch of the aircraft from the verticalgyroscope and the calculated pitch signal from said combining means, toform a blended pitch signal.

6. The pitch angle generator of claim 5 in which said summing meansrelates the calculated and measured pitch signals by factors of K and1-K, respectively, where K is a positive factor which is not greaterthan one.

1. A pitch angle generator for an aircraft, comprising: a longitudinalaccelerometer having an output which is a function of the forwardacceleration and the pitch attitude of the aircraft; a verticalgyroscope having an output which represents the measured pitch of theaircraft, subject to static and long time drift errors; first means forderiving from the output of the longitudinal accelerometer a signalwhich is a function essentially only of the forward acceleration and thelong term pitch attitude of the aircraft; second means for deriving fromthe output of the vertical gyroscope a signal which is a functionessentially only of short term pitch attitude of the aircraft; thirdmeans for developing a signal representing forward acceleration of theaircraft; and combining means for subtracting the forward accelerationsignal from the signal derived by the first means and adding said longterm and short term pitch attitude signals to generate a calculatedpitch signal.
 2. The pitch angle generator of claim 1 in which the meansfor developing a signal representing forward acceleration of theaircraft includes means for measuring the air speed of the aircraft andmeans for establishing a signal representing the rate of change of theair speed.
 3. The pitch angle generator of claim 1 in which the twosignal deriving means form a complementary filter having a low passfilter section connected with the output of said longitudinalaccelerometer and a high pass filter section connected with the outputof said vertical gyroscope.
 4. The pitch angle generator of claim 3 inwhich the means for developing a signal representing a forwardacceleration of the aircraft includes means for measuring the air speedof the aircraft and means, including a band pass filter section, forestablishing a signal representing the rate of change of air speed,which is combined with the output of said low pass filter section. 5.The pitch angle generator of claim 1 including means for summing asignal representing the measured pitch of the aircraft from the verticalgyroscope and the calculated pitch signal from said combining means, toform a blended pitch signal.
 6. The pitch angle generator of claim 5 inwhich said summing means relates the calculated and measured pitchsignals by factors of K and 1-K, respectively, where K is a positivefactor which is not greater than one.